U.S. Pat. No. 4,484,928, which is assigned to the assignee of the present application, discloses a state-of-the-art agglomeration-type coal cleaning process for generating product coals which have an extremely low ash content and essentially no pyritic or sulfatic sulfur.
This novel, and economically important, result is obtained by milling or otherwise comminuting raw coal until it has been reduced to a top size not greater than ca. 250 .mu.m.times.0 (.mu.m equals micrometer or micron). The raw coal is then slurried in an aqueous liquid, typically clean water; and comminution of the raw coal is continued until the raw coal has been resolved into separate, particulate phases of coal and mineral matter.
After this comminution step is completed: (1) the slurry is diluted to reduce its solids content to a maximum of 15 weight percent, based on the total weight of the slurry, and preferably to a solids content of 3-8 weight percent; (2) an agglomerating agent or agglomerant is added to the diluted slurry with agitation; (3) agitation of the slurry is continued until the coal particles have dissociated from the mineral matter and aqueous phases of the slurry and coalesced into agglomerates of product coal; and (4) the agglomerates are recovered from the slurry (there is virtually 100 percent recovery of the carbonaceous material in this separation).
A product coal with an even lower ash content than is available from following the steps identified above can be produced by redispersing the product coal agglomerates in clean water and repeating the agglomeration and collection steps. This sequence can be repeated as many times as wanted although it is presently believed that the benefits obtained by proceeding beyond the second or third collection step will in general not justify the expense of doing so.
No additional milling is required in the second product coal recovery stage (dispersion, agglomeration, and recovery steps) just discussed or in subsequent repetitions of this sequence of steps. Consequently, the elimination of additional mineral matter afforded by the second (and any subsequent) agglomeration stages can be effected inexpensively and with only modest expenditures of energy.
U.S. Pat. No. 4,186,887, also assigned to the assignee of the present application, is concerned with agglomeration processes much like those described in U.S. Pat. No. 4,484,928. The processes disclosed in the two patents differ in that, in those disclosed in the later issued patent, there is no milling of the raw coal during the recovery phase of the process; i.e., that phase in which the coal particles are separated from the aqueous phase of a coal-water slurry and coalesced into product coal agglomerates. This leads to a lower ash coal than could otherwise be produced by selective agglomeration of a raw coal.
Copending application Ser. No. 712,202, also assigned to the assignee of the present application, is similarly concerned with an agglomeration type process for recovering coal from mineral matter associated therewith. The process disclosed in that application however differs from the agglomeration type processes to which the '928 and '887 patents are devoted in that the raw coal being beneficiated is not milled or otherwise comminuted once it has been slurried. This requires that the coal being processed have a top size of not more than ca. 0.6 mm.
Coals processed as described in application No. 712,202 do not have the ultra low ash content of those beneficiated by the technique described in the '928 patent. Offsetting this, however, is the advantage that the cost of producing them is much lower because wet milling is not employed.
Unique in the processes described in the '928 and '887 patents and in application Ser. No. 712,202 is the use of an essentially pure compound as an agglomerant to effect a rapid and clean separation of the coal particles from the pa:ticles of mineral matter dispersed in the aqueous phase of the coal-water slurry.
The agglomeration steps of the coal cleaning processes disclosed in U.S. Pat. Nos. 4,484,928 and 4,186,887 are carried out in batch fashion in a mixer used to add agglomerant to the slurry and in a separator or reactor which may be a rotating drum or a spheroidizer. In the separator, the dissociation of the product coal from the mineral matter and aqueous phases of the slurry into which the raw coal is incorporated and the formation of product coal agglomerates, all initiated in the mixer, are continued and the agglomerates dimensionally stabilized; and water is expelled from the agglomerates, contributing to the quality of the product coal.
Because earlier initiated process steps--such as the milling of the coal to an appropriate size consist and the forming of the aqueous, coal-containing slurry--are carried out in continuous fashion, it becomes necessary to limit, and closely control, the residence time of the slurry in the reactor (i.e., the agglomeration time) to (typically) 60 seconds so that the reactor can accommodate the input of the slurry generated elsewhere in the system or accommodate high tonnage flows. Otherwise, it may become necessary to shut down the operations upstream of the separator from time-to-time; and this is uneconomical, if not impractical.
Similar considerations may make it necessary to limit and control the residence time of the slurry in a reactor in which an agglomeration type coal separation process as described in U.S. application Ser. No. 712,202 is being carried out.
Controlling or limiting the agglomeration time by mechanical measures is impractical because a change in agglomeration time entails changes throughout the coal cleaning system that are time-consuming and expensive to make and because seemingly inconsequential variations in the chemical make-up of the coal being processed can alter to a marked degree the time required for the separation and agglomeration of the product coal. Thus, if mechanical measures were relied upon to control residence time, plantwide changes might have to be made each time a different batch of coal was processed. We thus consider it impractical to mechanically alter the coal cleaning system to the extent necessary to produce an acceptable product every time a different coal is processed; and the ability to so process different coal is a desidaratum, if not a requisite, of a selective agglomeration type coal cleaning system.